1. Field of the Invention
The present invention relates to a multicast communication method to be performed in a CDMA (Code Division Multiple Access) mobile communication system.
2. Description of the Related Art
As a conventional multicast communication method in the CDMA mobile communication system, Japanese Patent Laid-Open Hei 10 (1998)-107770 discloses a multicast communication method in which service negotiations for the multicast communication is performed in a calling procedure between a mobile subscriber (abbreviated as MS, hereinafter) and a base station system (abbreviated as BSS, hereinafter) for the MS obtaining a spread code to be used for each multicast communication method.
Alternatively, in a plurality of BSSs, each multicast service data has been transmitted by using a broadcast channel of each BSS without any linkage to one another.
However, the following problems were inherent in the foregoing conventional multicast communication method in the CDMA mobile communication system.
(1) Regarding a system where a spread code is allocated for each multicast communication method by service negotiations in a calling procedure between the MS and the BSS, since a registration operation for the spread code is executed for each multicast service in MS that receives information, processing becomes complex. Thus, the system is not suitable for the multicast communication method of advertising use where information is simultaneously transmitted to a large number of MS. In addition, since the spread code must be fixed for each multicast communication method to be notified, dynamic use of limited number of spread codes is not allowed. Thus, this system lacks efficiency.
(2) Regarding a system where each multicast communication data is transmitted by each BSS by using respective broadcast channels, since the position of MS cannot be specified when a multicast operation is performed, transmission power in each BSS must be set to a maximum in order to enable possible MS located in the vicinity of a boundary to receive a signal sent from BSS. But if signal for data is transmitted by an output more than necessary, noises are generated in other MS. Consequently, a capacity of the entire system is reduced.
The present invention was made in order to solve the foregoing problems inherent in the prior art, and it is an object of the invention to provide a multicast communication method in a CDMA mobile communication system, which is capable of reducing transmission power in BSS and performing an efficient multicast operation for a large number of MS.